In this article, the performance of three different mixing elements on color dispersion in high‐density polyethylene and linear low‐density polyethylene polymer stream during extrusion is studied. Two similarly designed Maddock mixers and a Stratablend II mixer are used as the last part of a general purpose single screw. Moreover, an inline melt camera is used for the quantification of mixing quality by visualization of grayscale of the color dispersion and thus mixing. The Stratablend II mixer produces the lowest and most uniform standard deviation. Both the Maddock mixers showed the same trend but higher values of standard deviation. All results are then compared with a full 3D finite element method simulation. Simulations clearly indicate that the Stratablend II mixer has the best mixing abilities and that these are mainly given by its unique design with high average value of shear stress. The role of elongational stress does not appear to have a high influence on mixing for these mixers. The results also suggest that the key factor for achieving better mixing is the frequency by which a large fraction of the material passes through the high shear stress regions of the mixer. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers
One of the most important, yet problematic, issues in the extrusion process is achieving good mixing. Considerable prior efforts have been made to understand different types of mixing elements for single-screw and twin-screw extrusion. However, there is still a lack of good process values or criteria that can be used for design purposes. The focus of this work is to better quantify the mixing behavior, using 3D FEM analysis, to develop some design criteria. This study will focus on the fluted mixer, comparing common design variations and the effect of material viscosity and process conditions.
In this paper, the effect of the Maddock mixer design on its performance in the single screw extrusion of High-density Polyethylene (HDPE) melt has been investigated experimentally on a special extrusion line equipped with a barrel having several glass windows as well as theoretically using a three-dimensional finite element method simulation. The red-green-blue spectral analysis has been used for the experimental quantification of the mixing efficiency, whereas a generalised Newtonian model (with Carreau-Yasuda viscosity function) and a viscoelastic modified White-Metzner model have been utilised in flow simulations. Based on the performed experimental work, it has been found that the mixer with no undercut wiping flight ('Closed' mixer) has a faster mixing/ purging but a similar final mixing performance as the mixer with undercut wiping flight ('Open' mixer). It has also been revealed that the 'Open' mixer creates an unwanted dead zone (in the form of the stagnation layer of material rotating between the mixer and the barrel) at which the polymer melt degradation may take place. It has been found that based on the stress, residence time and pathline calculation, it is possible to predict the experimentally observed tendency of the 'Closed' mixer for a faster mixing/purging in comparison with the 'Open' mixer as well as the presence of a stagnation layer in the 'Open' mixer. The performed theoretical parametric study indicates that the gradual opening of the wiping gap causes the pressure driven flow to become more dominant than the drag flow in shearing as well as in the wiping gap independently of the utilised rheological model. On the other hand, the predicted particle trajectories inside the mixer were found to be shorter for the viscoelastic model, tending to occur mainly in the middle of the channel and thus leaving the mixer a little bit faster in comparison with the purely viscous calculations. Finally, it has been revealed that for the highest tested wiping flight opening, the viscoelastic modified White-Metzner model predicts back flow over the wiping flight, whereas the purely viscous Carreau-Yasuda does not, which can be explained by the elongational viscosity, considered in the viscoelastic modified White-Metzner model. This suggests that the modified White-Metzner model should be preferred more than the purely viscous Carreau-Yasuda model in the mixing element die design optimisation.
In this paper, a detailed three-dimensional finite element method (3D FEM) study of the mixing performance of the Fusion screw geometry by using two different rheological models is presented. Special criteria characterising the mixing performance in dependence of the barrier undercut separating the waving channels are developed. A great mixing performance is achieved when a right balance in both dispersive and distributive parts of the mixing process is found. An optimal undercut of the barrier was found to be about 2 mm. Both rheological models were successfully used to validate data of the real experiment with an error less than 15%.
The compression test provides a useful information about the material flow under large plastic deformations, when compared to the standard tensile test. On the other hand, it is heavily dependent on the lubrication conditions. The paper deals with two different specimen geometries resulting in two different friction conditions. The first geometry corresponds to the classical smooth cylinder. The second one was designed according to the idea of Rastegaev. It is a smooth cylinder with grooves on both faces (bases), which allow the lubricant to accumulate and prevent it from escaping the region, where the punches are in the contact with specimen. Then, both cases were computationally simulated and compared with experiments.
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